Earthquake Research in China  2019, Vol. 33 Issue (3): 478-488     DOI: 10.19743/j.cnki.0891-4176.201903009
Study on Dynamic Characteristics of Crustal Deformation in the Yunnan Area Using GPS
WANG Yan, SHAO Desheng, HONG Min     
Yunnan Earthquake Agency, Kunming 650224, China
Abstract: Based on the GPS velocity field data of 1999-2007 and 2011-2013, we used the least squares configuration method and GPS velocity profile results to synthetically analyze the dynamic evolution characteristics of crustal deformation in the Yunnan area before and after the Wenchuan earthquake. The dynamic evolution of GPS velocity field shows that the direction is gradually changed from the south in the southern part of the Sichuan-Yunnan block to the south-west in the southern Yunnan block and there is a clear relative motion characteristic near the block boundary fault zone. Compared with the GPS velocity of 1999-2007, the results of 2011-2013 also reflect segmental deformation characteristics of the block boundary fault zone. Southeast movement shows a significant increase, which may be related to crustal deformation adjustment after the Wenchuan earthquake. The dynamic evolution of strain parameters shows a pattern of "extension in the middle and compression at both ends" in the whole area and the distribution of deformation (shear, extension or compression) is closely related to the background motion and deformation characteristics of the main fault zone. Compared with the results of the period of 1999-2007, the extensional deformation zone of 2011-2013 is expanded eastward and southward. The compressional deformation of the eastern boundary (the Xiaojiang fault zone) of the Sichuan-Yunnan block is no longer significant, which is mainly concentrated in the northern section of the Xiaojiang fault zone and may be related to the post-seismic deformation adjustment of the Wenchuan earthquake. The GPS velocity profile results show that the left-lateral slip velocity of the Xiaojiang fault zone reduced gradually from north to south (10mm/a-5mm/a), and the width of the northern section is wider. The right-lateral slip rate of the Honghe fault zone is about 4mm/a, and the deformation width is wider. The dynamic results show that the Wenchuan earthquake has little effect on the deformation modes of these two fault zones.
Key words: Least squares configuration     GPS velocity field     Strain parameter     GPS velocity profile    

INTRODUCTION

Yunnan, located in the southeastern margin of the Qinghai-Tibetan Plateau, is one of the areas with the most significant seismic activities in China. Active structures are rich and complex in this area(Zhang Junchang et al., 1980; Wang Yizhao et al., 1988; Li Xi, 2015). This particular tectonic environment contributed to the special crustal deformation characteristics in this region. The study region is located in the southern section of the North-South Seismic Belt, where many researchers have conducted a lot of studies on the characteristics of crustal deformation along the North-South Seismic Belt(Jiang Zaisen et al., 2001; Fang Ying et al., 2008; Yang Guohua et al., 2009; Ding Kaihua et al., 2013; Zhang Xi et al., 2003; Wu Yanqiang et al., 2012; Wei Wenxin et al., 2012; Ji Lingyun et al., 2015). In addition, some scholars have done relevant studies on crustal deformation characteristics in Yunnan area(Yang Guohua et al., 2003; Hong Min et al., 2014; Wang Lingli et al., 2015). However, research on the dynamic evolution characteristics of crustal deformation in the Yunnan area is still insufficient. It's worth further researches after the impact of the Wenchuan MS8.0 earthquake in 2008 on the crustal deformation in the Yunnan area and if there are significant changes in regional deformation before and after the earthquake. Therefore, based on the GPS velocity field data during 1999-2007 and 2011-2013 in the Yunnan area, combined with the two periods of strain parameter results and GPS velocity profile results obtained by the least square collocation method, we synthetically analyzed the dynamic evolution characteristics of crustal deformation in the Yunnan area before and after the Wenchuan MS8.0 earthquake.

1 GPS DATA AND DATA PROCESSING

GPS observation in China began gradually since the late 1980s, and large-scale and high-precision GPS observation began after the completion of the "Crustal Movement Observation Networks of China", which covers about 1000 regional GPS stations in the Chinese mainland. On the basis of the Crustal Movement Observation Networks of China, the second-phase Crustal Movement Observation Networks of China(CMONC for short), which was launched in 2007, has increased the number of regional stations in the Chinese mainland to 2000. In order to study the dynamic evolution characteristics of crustal deformation in the Yunnan area, GPS remeasurement data after the completion of the Crustal Movement Observation Networks of China was used in this study. These include the remeasurement data of regional GPS network of the Crustal Movement Observation Networks of China in 1999, 2001, 2004 and 2007, and remeasurement data of regional GPS network of the second-phase Crustal Movement Observation Networks of China in 2011 and 2013.

The precise calculation of velocity field of regional GPS network is similar to that of GPS time series, which is carried out in two steps. The first step is the GAMIT calculation. Due to the large number of measurement points for synchronous observation and the limited number of measurement points for a single GAMIT calculation, the method of computational partitions is adopted, and the main output of this step is single-day relaxation solution hfile. In the second step, the single-day relaxation solution hfile is used as input to integrate the results of continuously operating GPS stations in the Chinese mainland and the results of IGS single-day relaxation solution, and GLOBK or QOCA software(Dong D. et al., 1998) is used to solve the GPS velocity. Since the calculation of GPS velocity requires at least two periods of GPS observation, it is particularly important to unify the basis reference for the computed results of two periods(multiple periods). The selecting principle of points of reference is similar to that of time series. Moreover, we should focus on the deduction of co-seismic displacement. If GPS observation has been conducted after an earthquake, the co-seismic displacement distribution model can be established by using the observed data to remove co-seismic effects.

Based on the above processing principles, GAMIT/GLOBK software is used to calculate GPS velocity field in the Yunnan area during the four periods from 1999 to 2007 and two periods from 2011 to 2013. Considering that the west side of south central North-South Seismic Belt is the South China Block-the largest relatively stable block in the Chinese mainland, the theoretical value determined by Euler rigid body motion parameters of the South China block is deducted from the measured velocity of GPS stations in the Yunnan area. So as to obtain the crustal movement velocity map of this region relative to the South China block is more appropriate. It can reflect the difference between the intense deformation of the Qinghai-Tibetan block to which the Yunnan area belongs and the stable South China block, and also reveals the relative movement of the main blocks along the boundary zone in the Yunnan area. In addition, aiming at the key problem that restricts the dynamic evolution characteristics analysis of multi-period GPS velocity field, we effectively solve the influence of datum shift of multi-period velocity field on gross error elimination by using the method of improved quasi accurate detection(QUAD) of primary index, and obtain the results of velocity field relative to the South China block(Zou Zhenyu et al., 2015). Fig. 1 provides the results of the GPS velocity field of the Yunnan area relative to the South China block in 1999-2007 and 2011-2013 calculated from the above GPS observation data.

Fig. 1 GPS velocity fields of the Yunnan area of different periods (relative to the South China Block)
2 RELATIVE CRUSTAL MOVEMENT IN YUNNAN AREA AND ITS DYNAMIC CHARACTERISTICS

The overall crustal relative movement and deformation characteristics in the Yunnan area(Fig. 1) reflect the fact that the material flow to the edge of the eastern Qinghai-Tibetan Plateau is blocked by the relatively stable boundary block, which resulting in crustal shortening near the border zone of the Qinghai-Tibetan Plateau in the perpendicular direction and crustal extension along the boundary of the block, with characteristics of both wrench and shear deformation. According to velocity attenuation or direction change, the movement is mostly in SE or south direction. The magnitude and direction of velocity vector of GPS stations in the Xiaojiang fault zone, as an important block boundary zone in this region, change constantly, showing strong relative motion and deformation. This indicates that due to its special tectonic environment and its eastern side is blocked by the stable block, its absorption of the eastward movement of the Qinghai-Tibetan Plateau and eastward moving of material is very significant. The Yunnan area is characterized by near NS crustal shortening deformation and near east-west crustal extension. The maximum shortening is about 8mm/a in NS direction within 800km from the southernmost region, while the crustal stretching is about 10mm/a in EW direction in the southernmost region. The most significant crustal movement and deformation in this area is the southward extrusion slip of the Sichuan-Yunnan block, and the velocity of movement in the middle part of the Sichuan-Yunnan block is the highest. The south as a whole shows a clockwise torsion motion. The velocity of GPS stations, from west to east and from north to south, mostly in the south direction, shifts to a south-west movement in the south of the Sichuan-Yunnan block. On the eastern edge of southern North-South Seismic Belt, that is, the components of GPS station velocity in the boundary zone of the Sichuan-Yunnan block and the South China block, which are parallel to the boundary zone, attenuate significantly from west to east, showing obvious sinistral deformation, and the overall sinistral deformation amount is about 10-12mm/a.

As can be seen from Fig. 1(a), the characteristics of GPS velocity field of 1999-2007 are mainly manifested as: (1) the southward GPS velocity field in the south of the Sichuan-Yunnan block shifted gradually to the SW direction in the south Yunnan block, revealing the deformation characteristics of clockwise rotation around the eastern Himalayan syntaxis; (2) there exists significant sinistral shearing motion in the southern section of the eastern boundary of the Sichuan-Yunnan block and a significant extensional deformation; (3) the material flowing east on the Qinghai-Tibetan plateau is blocked by the stable South China block, resulting in the existing deformation characteristics in the Yunnan area. Although the regional structure is very complex, the relative motion characteristics of the major block-boundary faults can still be clearly shown in the velocity field. As can be seen from Fig. 1(b), the density of GPS sites increased significantly during 2011-2013, however, due to the short time of observation, the error is greater than the results of 1999-2007. The results of velocity field reflect not only the relative motion characteristics in the border zone of blocks, but also the segmental deformation characteristics of the fault zone(e.g. the sinistral slip rate in the north and middle of the Xiaojiang fault zone is significantly higher than that in the south). In addition, compared with the results of velocity field of 1999-2007, there is a significant increase in the SE movement of the south Yunnan block and its surrounding areas during 2011-2013.

3 DYNAMIC VARIATION CHARACTERISTICS OF STRAIN RATE FIELD IN THE YUNNAN AREA

Although the relative motion velocity field can be used to intuitively reflect the characteristics of crustal deformation, the quantitative description of crustal deformation shows that the analysis of strain field is more advantageous. Various parameters of the strain field can fully express different properties and degrees of deformation. In general, the strain is determined by the relative motion between observation points, it does not depend on the reference datum. However, there are many methods for strain calculation at present, and the strain field images given by different researchers using GPS data from the same source but different approaches are also obviously different(Shi Yaolin et al., 2006). Wu Yanqiang et al. (2011) compared and analyzed the results of strain field obtained by the method of least square collocation with those of triangular element method, multi-quadric function fitting and spherical harmonics with the help of simulated experimental data, and objectively estimated the advantages and disadvantages of different methods, as well as their robustness and practicability, according to the degree of approximation to the theoretical value. The results show that the least square method is optimal. Therefore, the least square collocation spherical strain rate calculation method is used to analyze the characteristics of strain rate field in the study area (Jiang Zaisen et al., 2010).

Fig. 2 shows the distribution of surface strain rate and the maximum principal strain rate in two periods. The results of 1999-2007 indicate that: (1) extensional deformation is manly concentrated in the Honghe fault zone, Chenghai fault zone and the southern segment of Lijiang-Xiaojinhe fault zone, while compressional deformation is mainly concentrated in western Yunnan and the southern section of the eastern boundary of the Sichuan-Yunnan block, which on the whole shows the deformation characteristics of "extension in the middle, and compression at both ends"; (2) the region of extensional deformation is consistent with the deformation characteristics of fault zone obtained by geological results(Liu Guangxun et al., 1986; Huang Xiaojin, 2014), and the compressional deformation may be related to the dynamic source in this area, that is, the material flowing to the east of Qinghai-Tibetan Plateau is blocked by the stable South China block, forming a zone of compressional deformation in the southern section of the eastern boundary of the Sichuan-Yunnan block(the Xiaojiang fault zone); (3) a high value zone of compression deformation is formed in the boundary area of southern Yunnan, however, because of the boundary effect in the calculation process of the least square collocation method, the reliability of the high-value compressional deformation zone is not high. The results of 2011-2013, on the one hand, inherit the overall deformation characteristics of the precious period, on the other hand, there are significant differences, which are mainly reflected in the following aspects: the extensional deformation zone expands and tends to extend eastward and southward; Near the Zemuhe fault zone, there is a high value zone with significant compressional deformation, and the extrusion deformation along the eastern boundary of the Sichuan-Yunnan block(the Xiaojiang fault zone) is no longer significant on the whole, which is mainly concentrated in the northern part of the northern segment of Xiaojiang fault zone.

Fig. 2 Results of surface strain and principal strain rate of the Yunnan area in different periods

Fig. 3 shows the results of the maximum shear strain rate. The most distinguishing feature of the spatial distribution during the period of 1999-2007 is that the high value of strain rate is mainly distributed along the southern section of the eastern boundary of the Sichuan-Yunnan block(the Xiaojiang fault zone). In addition, the Chenghai fault zone is taken as the boundary to form a transition zone between high value and low value of the maximum shear strain rate, which may indicate a high level of shear stress accumulation in this region, and attention should be paid to the risk of strong earthquakes in this area. Besides, the distribution characteristics of regional shear deformation are closely related to background motion and deformation characteristics of major regional faults (For example, the Xiaojiang fault zone, characterized by left-lateral strike-slip movement, is in the high value zone of the maximum shear strain rate, while the Honghe fault zone, characterized by right-lateral strike-slip movement, is in the low value zone of the maximum shear strain rate). The spatial distribution characteristics during the period of 2011-2013 are consistent with that of the precious period, which also indicates that crustal deformation has good inheritance over time. Moreover, compared with the previous period, the high-value zone of the maximum shear strain near the Chenghai fault zone tends to expand northward. Meanwhile, the high-value zone of the maximum shear strain in the Honghe fault zone also tends to extend northward.

Fig. 3 Results of maximum shear strain and principal strain rate of the Yunnan area in different periods
4 STUDY ON MOTION CHARACTERISTICS OF MAJOR FAULT ZONES

Since the least square collocation strain rate calculation method is based on the assumption of continuous deformation, its results are not advantageous in identifying specific deformation patterns of specific fault zones. GPS velocity profiles can directly show the relationship between displacement distribution and faults, and are often used in analysis of the characteristics of fault deformation (Zhang Peizhen et al., 2005, 2008; Maurin T. et al., 2010; Wu Yanqiang et al., 2015). Therefore, the Xiaojiang fault zone and Honghe fault zone, are studied in this paper to analyze the dynamic evolution characteristics of motion and deformation of fault zones.

4.1 Dynamic Deformation Characteristics of the Xiaojiang Fault Zone

The Xiaojiang fault zone, SN-striking, starts from the north of Qiaojia in the north to the southeast of Jianshui in the south, with a total length of about 400km. According to its internal structure, it is divided into three sections, namely the north section, the middle section and the south section. In its historical process of activities, the fault has experienced transformation of different mechanical properties such as compression, extension and torsion, and since the late Quaternary, the fault is characterized by intensive left-lateral strike-slip movement(Song Fangmin et al., 1998). Based on the above segmentation principle, with the fault zone as the center, the left and right sides are delimited to a certain range respectively, and Gaussian projection is applied to the measured GPS velocity field data within the range to obtain the velocity of each GPS site that is perpendicular or parallel to the strike direction of the fault zone. Then the profile is fitted according to interseismic deformation patterns of the strike-slip fault, and finally the GPS velocity profile and fitting results, parallel to the strike of the fault zone, are obtained, as shown in Fig. 4.

Fig. 4 GPS velocity profiles in different sections of the Xiaojiang fault zone during 1999 to 2007 and 2011 to 2013

It can be seen from Fig. 4 that: (1) the slip rates of the northern section of the Xiaojiang fault zone have little change during the two periods, both of which are 10mm/a, presenting a deformation characteristic of strain accumulation, with wide deformation width; (2) the far-field slip rate in the middle section of the Xiaojiang fault zone in the period of 2011-2013 increased compared with that of 1999-2007(from 7mm/a to 10mm/a). The Xiaojiang fault zone is divided into two branches from the middle section. The existing density of GPS sites cannot identify the deformation characteristics of the east and west branches, and the obtained slip rate results should be the sum of slip rates of two branches. Although the slip rate increased and the profile still showed certain strain accumulation characteristics, the deformation widths in the two periods did not change much; The middle section has a narrower deformation width compared with that in the north section; (3) compared with the northern section, the slip rate of the southern section of the Xiaojiang fault zone is significantly reduced, about 5mm/a, which may be related to the absorption of partial deformation by the Qujiang-Shiping fault zone. According to the dynamic evolution characteristics of the two periods, there is no significant change in deformation width, and the deformation width is narrower than that of the northern section of the Xiaojiang fault zone. Based on the above analysis, it can be seen that the Xiaojiang fault zone is characterized by left-lateral strike-slip movement, deformation in different sections is obviously different, and slip rates are also different; according to the deformation mode of the strike-slip fault in the interseismic period, it is speculated that the Xiaojiang fault zone had a high level of strain accumulation; Besides, according to the dynamic evolution characteristics for each section, it can be seen that different sections of the Xiaojiang fault zone do not change obviously in different periods, thus co-seismic deformation of the Wenchuan earthquake and post-seismic adjustment have small impacts on this fault zone.

4.2 Dynamic Deformation Characteristics of the Middle Section of the Honghe Fault Zone

According to the geometric structure, the Honghe fault zone can be divided into three deformation zones, and there are several secondary faults in the northwestern and southeastern sections (Xiang Hongfa et al., 2004). Taking into account the actual distribution characteristics of GPS sites, only the middle section of the Honghe fault is analyzed with GPS profiles in this paper. Because the Honghe fault zone is dominated by right-lateral strike-slip movement. Fig. 5 provides the GPS velocity profiles and fitting results, parallel to the strike of the fault.

Fig. 5 GPS velocity profiles in the middle section of the Xiaojiang fault zone during 1999-2007 and 2011-2013

As can be seen from the GPS velocity profiles and fitting results, the middle section of the Honghe fault zone is dominated by right-lateral strike-slip movement with a slip rate of about 4mm/a, and the slip rates in the two periods have little change; The deformation pattern in the fault zone presents some features of strain accumulation. It can be seen from the fitting of the profiles of existing GPS sites that the deformation widths in the two periods have little change, but the deformation widths are relatively larger, which may indicate that this section may be in the late phase of earthquake preparation; it can also be seen from the above analysis that the occurrence of Wenchuan earthquake has a small impact on the deformation of the fault.

5 DISCUSSION AND CONCLUSION

Based on the GPS velocity field data of 1999-2007 and 2011-2013 in the Yunnan area, and combined with two periods of strain parameter results and GPS velocity profile results acquired by the least square collocation method, we synthetically analyzed the dynamic evolution characteristics of crustal deformation in the Yunnan area before and after the Wenchuan earthquake. The following conclusions are drawn:

(1) The dynamic results of GPS velocity field in the two periods show that the GPS velocity field, in the south direction, in the southern part of the Sichuan-Yunnan block, shifts to the south-west direction in the south of the Yunnan block, presenting the deformation characteristics of clockwise rotation around the eastern Himalayan tectonic syntaxis. There are obvious relative motions near the block-boundary fault zone. The density of GPS sites increased significantly from 2011 to 2013, and the velocity field results reflect not only the relative motion characteristics in block boundary zone, but also the segmental deformation characteristics of the fault zone. Compared with the results of GPS velocity field of 1999-2007, in south Yunnan block and its surrounding areas, there has been a marked increase in south-eastward movement.

(2) The dynamic results of the strain rate field in the two periods show the characteristics of "extension in the middle, and compression at both ends". The distribution characteristics of regional deformation (shear, extension or compression) are closely related to background motion and deformation characteristics of major regional faults. Compared with that of 1999-2007, the extensional deformation zone in the period of 2011-2013 expands and tends to extend eastward and southward. The compressional deformation along the eastern boundary of the Sichuan-Yunnan block(the Xiaojiang fault zone) is no longer significant on the whole, which is mainly concentrated in the northern part of the northern section of Xiaojiang fault zone. The high-value zone of the maximum shear strain near the Chenghai fault zone tends to expand northward; Meanwhile, the high-value zone of the maximum shear strain in the Honghe fault zone also tends to extend northward.

(3) GPS velocity profile across the fault zone shows that the left-lateral strike-slip rate of the Xiaojiang fault zone decreases gradually from north to south(reduced from 10mm/a to 5mm/a), and the deformation width is wider in the northern section. The right-lateral strike-slip rate of the Honghe fault zone is about 4mm/a with a wide deformation width. The dynamic results show that the Wenchuan earthquake has little impact on the deformation patterns of the two fault zones.

ACKNOWLEDGEMENT

We extend heartfelt thanks to research professor Wu Yanqiang's data solution team at the First Monitoring and Application Center, China Earthquake Administration, for providing technical support for GPS data processing in our study, and to Dr. Wei Wenxin from Institute of Earthquake Forecasting, China Earthquake Administration, for a useful discussion on GPS deformation analysis.

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